Armed police are continuing to search a house in Sunbury-on-Thames, Surrey.
The man was arrested in the departure area of Dover port on Saturday and has been moved to a London police station.
Thirty people were injured after the explosion on a train at Parsons Green.
The arrest is "very significant" and the terror threat level remains "critical", Metropolitan Police Deputy Assistant Commissioner Neil Basu said.
The arrest sparked an evacuation of Dover port and a number of items were recovered in a police search, Mr Basu said, adding that detectives were "keeping an open mind" as to whether more people were involved in the Tube attack.
Residents of houses surrounding the Sunbury-on-Thames property have been allowed to return home after being evacuated by officers earlier on Saturday.
Locals said the property being searched by police is occupied by married couple Ronald Jones, 88, and Penelope Jones, 71.
The couple are known for fostering many children and young people over several decades, and in 2010 were rewarded by the Queen by being made MBEs.
keyboard shortcuts: V vote up article J next comment K previous comment